Partial response systems (PRS) are well-known in the art. Partial response techniques (also called duobinary or correlative level encoding) are used to increase the signaling rate in a given bandwidth, "The Duobinary Technique for High-Speed Data Transmission" by A. Lender, IEEE Transactions on Communications and Electronics, May 1963, pp. 214-218, and "Generalization of a Technique for Binary Data Communication," by E. R. Kretzmer, IEEE Transactions on Communications Technology, February 1966, pp. 67-68.
These techniques have been applied to various forms of amplitude modulation using baseband signal processing to detect the multiple levels produced by the partial response process. One application has been in two dimensional, e.g. quadrature amplitude modulated (QAM), systems where each channel is independently amplitude modulated. When two level (binary) input signaling is used on each of the two channels before partial response filtering, a three level output is generated on each channel and is commonly referred to as QPRS modulation, "Modulation Considerations for a 91 Mbit/s Digital Radio" by C. W. Anderson and S. G. Barber, IEEE Transactions on Communications, May 1978, pp. 523-528. When the two quadrature channels are each encoded with four input levels before filtering, a QPRS system with seven output levels on each axis results, "A Four Bits/Hertz Radio at 8 GHz" by J. Alexander, R. Cheung and T. Kao, International Conference on Communication (IEEE), 1979, p. 5.7.1-5.7.5. Partial response techniques have thus been applied to single dimensional AM (amplitude modulation) and to two dimensional QAM systems.